The term is flexible enough to cover 95% of what we think of as noir, and it works well for French poetic realism, which--as we will see in greater detail when a complete history of French noir is (finally) in hand--hopefully by late spring...we keep finding items to add to the filmography--is but the privileged sub-type which is drawn from a welding together of exotic adventure tales and the collision of colonialism in conjunction with the charismatic villain (paging Jean Gabin).
Crime, corruption, guilty knowledge, alienation, marital/sexual discord--all ingredients in a semi-improvised recipe that allows for self-consciousness (a pervasive feature of modernism) that adds irony to sordid tales involving one or more of the "deadly sins." It all comes together in 1931-32, when sound has been added into film, permitting tone of voice to be embodied in an actor's performance, allowing us to become radically skeptical about what we see even as we are tracking the action. It pops for the first time in Renoir's LA NUIT DU CARREFOUR, which shows the murky, chained world of criminal conspirators with an interest in hiding their activities--in contrast with American gangsters of the same time frame, who project a megalomaniacal glee in self-advertisement. Noir doesn't need a classical mystery at its center to lean strongly towards the clandestine. And the gothic noirs are often the most scrupulous in terms of undetectable manipulation and emotional violence.
But here, on the hard-boiled edge, we have mostly id-like behavior patterns that create a repressive atmosphere and a level of amorality that demands a body count...